1. Field of the Invention
The present invention relates to condenser microphones, which are manufactured by way of semiconductor device manufacturing processes and are adapted to MEMS (micro-electromechanical system), and in particular to condenser microphones in which diaphragm vibrating due to sound waves are arranged opposite to plates so as to generate electric signals in response to variations of electrostatic capacitance therebetween.
2. Background Art
Conventionally, various types of condenser microphones manufactured by way of semiconductor device manufacturing processes have been developed. A conventionally-known condenser microphone is constituted in such a way that a diaphragm having a moving electrode, which vibrates due to sound waves, is arranged opposite to a plate having a fixed electrode, wherein the diaphragm and the plate are distanced from each other and are supported via an insulating spacer. That is, a condenser (i.e., electrostatic capacitance) is formed by means of the diaphragm and the plate, which are arranged opposite to each other.
In the aforementioned condenser microphone, when the diaphragm vibrates due to sound waves, the electrostatic capacitance varies due to the displacement thereof, so that variations of the electrostatic capacitance are converted into electric signals. The sensitivity of the condenser microphone increases when the ratio of the displacement of the diaphragm to the distance between the oppositely arranged electrodes is increased, i.e., by improving the vibration characteristics of the diaphragm. In addition, the sensitivity of the condenser microphone increases when the parasitic capacitance that does not contribute to variations of the electrostatic capacitance is decreased.
The paper issued by the Japanese Institute of Electrical Engineers and entitled “Mechanical Properties of Capacitive Silicon Microphone” teaches a condenser microphone in which a diaphragm and a plate are formed using conductive thin films. Herein, a spacer is fixed to the overall periphery of the diaphragm; hence, when sound waves are transmitted to the diaphragm, a relatively large displacement occurs in the center portion of the diaphragm, while a very small displacement occurs in the periphery of the diaphragm. As a result, vibration at the center portion of the diaphragm is efficiently detected as capacitance variations, while only the parasitic capacitance occurs in the periphery of the diaphragm. The parasitic capacitance reduces the sensitivity of the condenser microphone.
Japanese Patent Application Publication No. H09-508777 and U.S. Pat. No. 4,776,019 teach condenser microphones in which vibration characteristics of the diaphragm are improved by use of spring structures for supporting diaphragms so as to improve sensitivities. Specifically, slits are formed in the diaphragm, and spring functions are applied to regions defined by the slits. However, since the plate is arranged to entirely correspond to the diaphragm having the spring function, a parasitic capacitance occurs in a region causing small displacement due to vibration of the diaphragm, whereby the sensitivity of the condenser microphone decreases.
Japanese Patent Application Publication No. 2004-506394 teaches a condenser microphone, in which a plate arranged opposite to a diaphragm having a moving electrode is formed using an insulating material, and a rear electrode is arranged only in the prescribed portion of the plate positioned opposite to the center portion of the diaphragm, so that variations of electrostatic capacitance are efficiently detected in correspondence with the center portion of the diaphragm, thus reducing the parasitic capacitance at the periphery of the diaphragm and thus improving the sensitivity. However, since the rear electrode is arranged only in the prescribed portion of the plate positioned opposite to the center portion of the diaphragm, the manufacturing process becomes complex and the manufacturing yield decreases, thus increasing the manufacturing cost. When a gap is formed by removing a sacrifice layer intervened between the diaphragm and the plate by way of etching, the insulating material for fixing the plate and the rear electrode should be slightly etched. The countermeasure coping with this problem must be incorporated into the manufacturing process, which further increases the manufacturing cost.
The sensitivity of the condenser microphone depends upon the vibration characteristics of the diaphragm, the parasitic capacitance formed between the diaphragm and the back plate, and the rigidity of the back plate: hence, the prior-art technology for improving the sensitivity of the condenser microphone has problems in that structural complexity and operational instability occur, and the manufacturing yield becomes low due to the complex manufacturing process.
For example, it is possible to adopt a countermeasure in which, in order to reduce the parasitic capacitance, a plurality of small holes are formed in the region of the back plate positioned opposite to the periphery of the diaphragm so as to reduce the substantially opposite area therebetween; however, this reduces the mechanical strength of the back plate and increases the unwanted deformation of the back plate. In addition, it is possible to form projections so as to control excessive vibration of the diaphragm, whereby even when excessive sound pressure is applied to the diaphragm, or a mechanical impact is applied to the condenser microphone from the exterior, it is possible to prevent the diaphragm from coming in contact with the rear electrode arranged in the prescribed portion of the back plate. However, this requires a complex process for forming the rear electrode on the back plate composed of the insulating material, which reduces the manufacturing yield and increases the manufacturing cost.